Networks consist of a plurality of interrelated nodes. Packet switched networks integrate frame-delimited data traffic, packetized speech and constant bit rate (non-delimited) traffic onto common transmission facilities. The basic unit of transmission in such a network is a "fast packet", also referred to as a "cell". These fast packets are transmitted from node to node within the network to the packets' destination.
When data enters the network through a node, the data is segmented into "fast packets". A fast packet contains a header and a payload. The header usually includes connection identification and other overhead information. The payload contains the data. The size of fast packets is defined by the network. Thus, the header and the payload of the fast packets are also defined by the network. A number of fast packets are usually required to transmit the data through the network.
After the data is packetized, the packets are enqueued at the node with fast packets from other data sources and then transmitted through the network via internodal link transmission facilities. The fast packets are received at another node, and, if the node is not the ultimate destination of the fast packets, then enqueued again with fast packets from other data sources.
Eventually, the fast packets are received at their ultimate destination. The packets are then depacketized and "played back" to the end user (which could be, for example, a data terminal, storage device or audio device).
Fast packets do not move within the network at a constant rate. Fast packets may be delayed due to the various enqueuing procedures at the network nodes. For example, the number of other fast packets received and transmitted at each node impacts the time for transmitting the packets through the network. As failure of nodes or links in the network occur, the route of the fast packets from the inception node to the reception node may change. Packets may even be lost in the network.
Constant bit rate (CBR) data (commonly generated in synchronous communication) presents a special problem. Constant bit rate data is information generated at uniform periods. In order for the CBR data to be accurately processed, the data should be received at its destination at the same constant rate.
Because fast packets do not move within the network at a constant rate, the fast packets are not regularly received at the destination node. Further, the delay in the network may change over time. Provision must be made at the playback of the CBR fast packets to compensate for any irregularity.
One method is to delay the start of playback of the first CBR fast packet for a fixed time period. This may result in a number of CBR fast packets collecting prior to starting playback of the first CBR fast packet. The time delay does somewhat smooth the variable network delays playback of the CBR packets. However, such a method is inflexible and does not adapt to changes in the network. Further, there is no provision for handling of lost packets.
Another method is to "time stamp" each packet by including information in the payload of the packet as to when the packet was created. However, this method requires synchronized clocks at the network nodes. This may not be feasible in all networks, especially geographically large networks.